Saturday, April 18, 1998 Published at 03:46 GMT 04:46 UKSci/TechSpace station countdown under wayThe station is the most complex engineering project ever undertaken

The American space agency Nasa is about to start building a space station in orbit, 240 miles above the Earth.

Skylab was launched in 1973

The international space station, to be built with the collaboration of 16 nations including the Russians and the European Space Agency, will consist of 100 different modules and parts and will take six years to assemble.

June sees the launch of the first segment when the Russians send up the functional cargo block. It will be followed a month later by an American section, originally called Node 1 but now renamed 'Unity'.

It will need 45 space shuttle launches to lift the station into space, and more than 1,000 hours of space walks to put it together.

America's first space station, Skylab, was launched in 1973 and was built from an empty rocket casing.

Astronauts used it to see how they adapted to weightlessness in space.

The station will be twice the size of a jumbo jet

The new space station will be much more sophisticated, and will have space inside equivalent to two jumbo jets.

Scientists admit that putting the modules together will be a challenge.

The Director of Operations at Nasa, William Bates, said: "We don't have the luxury of being able to put all those together on the ground. It's a very complex undertaking."

Nasa's Space Station Research Manager, John-David Bartoe, said: "I personally believe that we have some research surprises ahead of us and that we will find some things that we never expected."

But many scientists remain sceptical, arguing that many of the experiments done in space can be done cheaply and more effectively on Earth.

The sections will be bolted together

Robert Park, of the American Physical Society, said: "The impact will be very, very slight. And compared to the cost it is hard to imagine that we could spend our dollars on research in a less efficient way."

Nasa maintains that the project is vital for at least one longer-term project - a manned mission to Mars.

The BBC's science correpondent James Wilkinson says compared to that, even the cost of the space station itself pales into insignificance.